This invention relates to classifiers and more particularly, it concerns an improved classifier construction which is adjustable to regulate the relative proportions of accepted and rejected solid particles suspended in a continuously flowing fluid stream.
In many industrial processes, there is a need for the separation of fluid-suspended solid particles on the basis of particle size or mass while the particles are in route to a point of use, application or treatment. For example, in the operation of power plants using pulverized coal for fuel, it is common practice to supply raw coal to a pulverizer in which multi-sized, relatively small particles are entrained in an air stream for supply as a combustible mixture. In its passage from the pulverizer, the air suspension of particles is passed through a classifier in which acceptable or relatively fine particles are allowed to pass from the classifier for combustion whereas unacceptable relatively coarse particles are rejected and returned to the pulverizer for further reduction in size. Such processes are usually continuous and, as such, restrict the range through which any one operation or step in the overall process may be varied without affecting other operations or steps.
A typical classifier for separating air-suspended particles in an industrial process may be in the nature of a heart-shaped enclosure having at its base, concentric ducting for feeding granular material such as raw coal and air to a ball mill pulverizer, for example, and for returning air-suspended particles to the enclosure. By appropriate baffling, the air suspension of particles is first directed upwardly so that a substantial portion of oversized particles will return by gravity to the pulverizer inlet ducting. As the air stream proceeds, it is passed in divergent arcuate or scroll-like paths enroute to discharge openings in the enclosure. By centrifugal force, unacceptably coarse particles move out of the air stream against the arcuate interior surfaces of the enclosure and fall to the bottom also for return to the pulverizer. Acceptably fine particles remain in the air stream and pass from the classifier to a point of use.
Although the relative percentages of particles passing from the classifier as against rejected particles returned to the pulverizer may vary with the velocity, temperature, and moisture content of the fluid or air stream as well as with particle size and shape, particle density and the like, the design of a classifier for a given process is dependent primarily on the size of ducting defined by the classifier enclosure and the radius of curvature through which the particle suspension is caused to pass. These latter parameters are usually fixed in prior art classifiers with the result that control over particle separation or classification is relatively restricted in a given installation.
In the operation of prior art classifiers of the aforementioned type in a pulverized coal burning system, there is a tendency for acceptably sized or fully pulverized particles of coal to collect on the interior surfaces of the arcuate walls defining the curved path for the air/coal suspension. Because the air velocity on the outside of the ducting or at the interior arcuate surface is relatively low, the fine particles will not be removed by the air but rather will tend to flake off from the surfaces as agglomerate chunks or particles of sufficient size and weight to cause their return to the pulverizer. This characteristic, in turn, has resulted in unnecessary or excessive repulverization and corresponding loss of efficiency not only of the pulverizing step but of the overall process. While some measure of regulation is afforded by varying the temperature and velocity of the air stream passing through the pulverizer, these parameters of operation effect other steps in the overall process so that meaningful variation in the classifier involves a trade-off with other operational factors.
There is a need, therefore, for improvement in classifiers of the general type described so that controlled operation of the classifier may be effected independently of other variables required for the process or system in which the classifier is used.